Compressor for a refrigerating machine

ABSTRACT

A compressor of the swash plate type for a refrigerating machine in which an oil separating chamber is formed in one portion of each of a plurality of cylinder block elements, and two sets of balance holes are formed therein, one set disposed between the oil separating chambers and a crank-case and the other set disposed between the oil separating chambers and a low pressure gas passage. By this arrangement, blow-by gas is ejected at high speed from the crank-case into the oil separating chambers, impinges on walls of the oil separating chambers, and changes its directions of movements to flow into the low pressure gas passage. During this process, lubricant is separated from the blow-by gas and recovered for further use.

United States Patent 1 Oshima et al.

1 COMPRESSOR FOR A REFRIGERATING MACHINE [75] Inventors: Ryoichiro Oshima; Atsushi Suginuma, both of Hitachi; Atsuo Kishi, Katsuta; Kenichi Kawashima, Hitachi. all of Japan [73] 'Assignee: Hitachi, Ltd., Japan [22] Filed: Sept. 25, 1973 [21] Appl. No.: 400,653

1 1 June 10, 1975 Primary Examiner-William L. Freeh Assistant Examiner-Gregory Paul LaPointe Attorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT A compressor of the swash plate type for a refrigerating machine in which an oil separating chamber is formed in one portion of each of a plurality of cylinder block elements, and two sets of balance holes are formed therein, one set disposed between the oil separating chambers and a crank-case and the other set disposed between the oil separating chambers and a low pressure gas passage. -By this arrangement. blowby gas is ejected at high speed from the crank-case into the oil separating chambers. impinges on walls of the oil separating chambers. and changes its directions of movements to flow into the low pressure gas passage. During this process. lubricant is separated from the blow-by gas and recovered for further use.

11 Claims, 8 Drawing Figures PATENTEIJJUN 10 I975 SHEET l (PRIOR ART) FIG.

FIG. 2

PO DJ 00 DU PATENTED JUN I 0 I975 SHEET 1 .813, PATENTEDJuNmms SHEET 4 o (504 COMPRESSOR FOR A REFRIGERATING MACHINE BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention relates to compressors for refrigerat ing machines. and more particularly it is concerned with a mechanism for precluding the carry-over of oil for a reciprocatory piston compressor of the swash plate type.

2. DESCRIPTION OF THE PRIOR ART In a reciprocatory piston compressor used with a cooling device. air conditioner. refrigerating machine and the like. rotary movements of the drive shaft are converted into reeiprocatory movements of the pistons in the crank-case so as to thereby compress a refrigeram in a gaseous state and move the same through various phases of the refrigeration cycle. The refrigerant in the gaseous state tends to leak through gaps between the pistons and cylinders (blow-by gas) into the crankcase and cause a rise in pressure therein. To obviate this disadvantage. a balance hole is provided to maintain the crank-case in communication with a suction side passage of the compressor to return the blow-by gas to the refrigeration cycle.

On the other hand, a lubricant or oil chamber is provided in a lower portion of the crankcase so as to supply oil to various sliding parts of the compressor to effect lubrication by splash lubrication or forced feed lubrication relying on an oil pump. As a result, a high negative or sub-atmospheric pressure acts on the crank-ease through the balance hole when the compressor is started. This would cause the lubricant to flow into a low pressure gas passage (the carry-over oil) due to oil foaming and move through the refrigeration cycle with the refrigerant. The lubricant would also flow into the refrigeration cycle through the gaps between the pistons and cylinders, so that shortage of lubricant in the lower portion of the crank-case would ensue.

Means has hitherto been provided to separate the lubricant from the refrigerant at the suction side or discharge side of the compressor and recover the same from the refrigeration cycle. In one device for separating the lubricant from the refrigerant known in the art, for example, the sectional area of the low pressure gas passage (suction side) is partly increased or the lubricant is required to change the directions of its movement to separate the lubricant from the refrigerant, and the increased sectional area portion of the low pressure gas passage is made to communicate with the crankcase through a balance hole. so that the lubricant can be returned (by gravity) and the blow-by gas can be reduced (by differential pressure).

Some disadvantages are associated with the aforementioned device. When the compressor is started, the carry-over of a large quantity of lubricant takes place owing to oil foaming, and only a small portion of such lubricant is recovered due to the fact that the lubricant flows in a direction opposite to the direction of flow of the blowby gas.

In another lubricant separating device known in the art. a lubricant recovery mechanism and a blow-by gas reducing mechanism are provided separately. The dcvice of this type also has disadvantages in that recovery of the lubricant is a time-consuming process and the construction of the device is complex.

SUMMARY OF THE INVENTION This invention has its object the provision of a compressor which is capable of precluding a rise in pressure in the crank-case. which permits lubricant to be recovered at early stages, and which is simple in construction.

The main characteristic of the invention is that oil separating chambers are formed between the crankcase and the low pressure gas passage and two sets of balance holes are formed therein, one set disposed between the oil separating chambers and the crankcase and the other set disposed between the oil separating chambers and the low pressure gas passage to maintain communication therebetween.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a conventional compressor of the swash plate type provided with a balance hole;

FIG. 2 is an external view ofthe cylinder block of the compressor shown in FIG. 1;

FIG. 3 is a vertical sectional view of the compressor of the swash plate type comprising one embodiment of the invention;

FIG. 4 is an external view of the cylinder block of the compressor shown in FIG. 3. illustrating the stream of a refrigerant;

FIG. 5 is a detailed view of a portion of the cylinder block shown in FIG. 4 at which a valve plate is attached to the cylinder block;

FIG. 6 is the valve plate joining portion shown in FIG. 5 as seen from the side of the valve plate; and

FIG. 7 and FIG. 8 are vertical sectional views of the compressors of the swash plate type comprising other embodiments of the invention.

DESCRIPTION OF THE EMBODIMENTS Before describing the invention in detail, I would like to briefly describe a conventional compressor of the swash plate type provided with a balance hole with reference to FIGv l and FIG. 2. l and 2 designate a cylin der block split into two pieces or rear and front cylinder block elements by a line perpendicular to the longitudinal axis of the cylinder block. 3 and 4 designate cylinder heads arranged in series at the front and rear ends of the two cylinder block elements I and 2 respectively. These elements and a shell 5 are firmly fastened together while being arranged in predetermined relative positions. 6 and 7 designate valve plates interposed between the cylindcr rear block element I and rear cylinder head 3 and between the front cylinder block element 2 and rear cylinder head 4 respectively, and disposed in predetermined relative positions. 8 designates a drive shaft connected. for example. to a drive of the engine of the vehicle and extending through the central portions of the cylinder block elements I, 2, front cylinder head 4 and valve plates 6, 7. The drive shaft 8, which is journalled by needle bearings 9 at its outer end portions, has mounted thereon an inclined disc 10 disposed near a position in which the two cylinder block elements I, 2 are joined together.

The two cylinder block elements I, 2 are formed with six axial bores 11 of three sets alined with one another and disposed parallel to the center axis of the drive shaft 8., and a reciprocatory piston 12 is received in each set of the bores. Each piston I2 has one side edge cut off as if to straddle an outer edge of the inclined disc 10, and the cut-off portion engages opposite sides of the inclined disc through a steel ball 13 and a slipper 14. so that the pistons 12 can move in reciprocating movement as the inclined disc 10 moves in rotary movement. An axial load applied by the compressing action of the pistons 12 is borne by two thrust bearings 15 interposed between opposite end surfaces of a boss of the inclined disc 10 and the two cylinder block elements 1, 2.

A segmental portion disposed above the drive shaft 8 and defined by an upper circularly arcuatc portion of the cylinder block and wall surfaces of two adjacent bores 11 constitutes a portion of a low pressure gas passage 16 through which a refrigerant in a gaseous state is led from the rear cylinder head 3 to the front cylinder head 4. The passage 16 has opposite ends maintained in communication with low pressure chambers 43 in the two cylinder heads 3, 4 through circular or segmental openings formed in the valve plates 6, 7 respectively. Another segmental region 40 (see FIG. 2) constitutes a high pressure gas passage and is maintained in communication with high pressure chambers 41 in the cylinder heads 3, 4 through openings formed in the valve plates 6, 7 respectively. Discharge orifices l7, l8 communicating with the segmental region 40 are connected to an outlet port (not shown) after extending through the interior of the shell 5. Another segmental region 42 disposed in the bottom portion defined by wall surfaces of the bores 11 has inserted therein an oil pump 21 which maintains an oil chamber 19 in the shell 5 in communication with a suction port ofa gear pump 20 for supplying lubricant.

The lubricant circulation system ofthe compressor of the type described will be explained. The oil in the shell oil chamber 19 is drawn by suction, pressurized and de' livered by the gear pump 20 to the needle bearings 9, thrust bearings 15, and a mechanical seal 23 through an axial oil bore 22 formed in the central portion of the drive shaft 8 and radial ducts 43a connecting the bore 22 with various sliding parts requiring lubrication. Lubrication of sliding interfaces between the steel balls 13 and seats of the pistons, between the steel balls 13 and slippers 14, and between the slippers 14 and inclined disc 10 is effected by oil ejected through the thrust bearings 15. The oil is returned to the oil chamber 19 after effecting lubrication.

During the compression stroke of each piston 12, the refrigerant in a gaseous state may leak through the gap between each piston 12 and its cylinder bore 11 into a crank-case 24 (such gas is hereinafter referred to as blow-by gas). The blow-by gas would flow from the crank-case 24 into the low pressure gas passage 16 through a small aperture generally referred to as a balance hole through which the crank-case 24 is maintained in communication with the low pressure gas passage 16. As the blow-by gas flows into the low pressure gas passage 16, oil scattered in the crank-case 24 as the inclined disc 10 rotates and adhering to inner walls thereof in film form or oil flying in all directions in the crank-case 24 would flow out of the crank-case 24 and move into the low pressure gas passage 16 together with the blow-by gas. Thus such oil moves out of the compressor.

The low pressure gas passage 16 includes a relatively narrow passage portion 16 and relatively wider passage portions 26, 26'. so that the refrigerant containing oil returning to the compressor after moving through the refrigeration cycle flows through the low pressure chamber 43 and a suction port 27 of a relatively small cross-sectional area formed in the valve plate 6 into the low pressure gas passage portion 26 of a relatively large cross-sectional area. Thus oil drops are ollected by gravity on the bottom of the relatively wide passage portion 26. Besides, the refrigerant impinges on a wall 28 of the cylinder block and the oil is separated therefrom. with the oil being also collected on the bottom of the relatively wide passage portion 26. The oil collected in this way finds its way through the balance hole 25 into the crank-case 24.

Because of the construction that the crank-case 24 is maintained in direct communication with the low pressure gas passage 16 as aforementioned, the oil flowing into the low pressure gas passage together with the blow-by gas is large in quantity in compressors of this type. Particularly, when the compressor is started, the carry-over of oilwould take place owing to oil foaming caused by gasification of the refrigerant in a liquid state dissolved in the oil in the oil chamber 19. This is largely responsible for the abnormal noise produced when the compressor is started or the breakdwon of the valve due to abnormal pressure. Also, the system of returning oil to the oil chamber relying on the balance hole is low in efficiency in recovering oil from the refrigeration cycle. This causes lack of lubrication and resultant seizure of the sliding parts. Moreover, such compressor is also low in the degree of efficiency with which refrigeration is effected because of a high content of oil in the refrigerant.

The aforementioned disadvantages of the prior art are obviated by the present invention which contemplates the provision of oil separating chambers between the crank-case and the low pressure gas passage. One embodiment of the mechanism for precluding the carry-over ofoil according to the invention will now be described.

In FIG. 3, 29, 29' designate oil separating chambers each of which is a segmental section disposed between the adjacent two bores 11 formed in the cylinder block elements 1, 2 and above the drive shaft 8. Each oil separating chamber 29 (29) is defined by an outer wall 24a (24b) of the crank-case 24, a wall 16a (16b) of the low pressure gas passage 16 and the valve plate 6 (7). 25, 25' designate a first set of balance holes formed between the oil separating chambers 29, 29 and the crank-case 24, and 30, 30' designate a second set of balance holes (grooves or small apertures) formed between the oil separating chambers 29, 29' and the low pressure gas passage 16. The relative positions of the parts described are shown in FIG. 4, FIG. 5 and FIG. 6. 31 designates an oil return opening formed between the rear oil separating chamber 29 and the needle bearing portion, and 32 designates an oil return opening formed between the front oil separating chamber 29' and the mechanical seal box.

Referring to FIG. 3 again, the blow-by gas leaking through gaps between the cylinder bores 11 and pistons 12 during the compression stroke of the latter is relatively high in pressure. Thus, because of a difference in pressure between such blow-by gas and the gas in the low pressure gas passage 16, the blow-by gas moves from the crank-case 24 through the first set of balance holes 25, 25' into the oil separating chambers 29, 29' together with a portion of the lubricant in the crankcase 24. Portions of oil drops are collected by gravity on the bottoms of oil separating chambers. However, the oil which is not separated from the blow-by gas impinges on the valve plates 6, 7, whereby the oil is separated from the refrigerant and also collected on the bottoms of the oil separating chambers 29, 29'.

Preferably, the second set of balance holes 30, 30' is disposed as far away as possible from the first set of balance holes 25, 25' in order to increase the efficiency of separating oil from refrigerant by gravity. Preferably. the second set of balance holes 30, 30' is disposed near the portions of the compressor at which the valve plates 6, 7 are mounted thereon in order that oil may be separated from refrigerant with a high degree of efficiency by impinging on the valve plates 6, 7. It is desirable that the balance holes 30, 30' of the second set and the balance holes 25, 25' of the first set should be constructed such that the former has a larger sectional area than the latter so as to increase the rate of flow of the oil containing gas through the balance holes 25, 25' over that of the oil containing gas through the balance holes 30, 30', in order that the degree of efficiency with which oil is separated from gas by the impinging of oil drops on the valve plates may be increased.

Caution should be exercised in selecting the sectional areas of the balance holes of the first and second sets. If the sectional area of the second set of balance holes 30, 30' were too large as compared with that of the first set of balance holes 25, 25', the streams of gas-oil mixture flowing through the balance holes 25, 25' would directly move into the low pressure gas passage 16 without impinging on the valve plates 6, 7. It is thus necessary that the sectional area of the first set of balance holes 25, 25' should be reduced as much as possible to increase the rate at which the gas-oil mixture is ejected thcrethrough while maintaining the size of the sectional area of the second set of balance holes 30, 30 at a suitable level, in order to effectively separate oil from blow-by gas and preclude the carry-over of oil due to the action of blow-by gas.

An example of the invention will be described with reference to FIG. 3. A compressor of the type shown in the figure with a capacity of about I70 cubic centimeters was operated at 3,000 rpm after setting the diameters of the balance holes and the data of relevant parts at suitable levels. The rate of flow of the blow-by gas through the balance holes 25, 25 was about 24 meters per second. It was ascertained experimentally that since the blow-by gas was caused to impinge on the valve plates 6, 7 at such high speed and change its directions of movement suddenly, almost all the oil was separated from the blow-by gas and moved downwardly along the valve plates. After having the oil separated in this way, the blow-by gas moved from the oil separating chambers 29, 29' through the second sets of balance holes 30, 30 into the low pressure gas passage 16.

Preferably, the first set of balance holes 25, 25 is formed near the upper wall of the crank-case 24. The major portion of lubricant scattered by centrifugal forces in the crank-case 24 as the inclined disc rotates adheres to the inner peripheral wall surface of the crank-case. Because of this, it would be possible to increase the efficiency of delivering the oil containing refrigerant to the oil separating chambers 29, 29' if the balance holes 25, 25 were disposed as aforementioned.

It is convenient from the point of view of machining that the balance holes 25, 25 are disposed at right angles to the valve plates 6, 7 respectively. However, the balance holes 25, 25' may be slightly downwardly oriented in order to increase the efficiency of separating oil from blow-by gas. As shown in FIG. 5 and FIG. 6, a recess 44 is formed in a portion of the low pressure gas passage 16 to facilitate movement ofa stream of refrigerant in a gaseous state from the rear low pressure chamber 43 into the low pressure gas passage 16. This is made necessary because of the fact that the second set of balance holes 30, 30' should be disposed at the same or a higher level than the first set of balance holes 25, 25'.

The oil collected on the bottom of rear oil separating chamber 29 moves by gravity through the oil return opening 31 to the needle bearing 9 portion from which it is returned to the crank-case 24. The oil collected on the bottom of front oil separating chamber 29' is moved by gravity through the oil return opening 32 to the mechanical seal box from which the oil is passed to the needle bearing 9 portion to lubricate the same before returning to the crank-case 24.

The oil not separated from the blow-by gas and the gas finding its way through gaps between the shell 5 and cylinder block elements 1, 2 into the low pressure gas passage 16 moves through the refrigeration cycle.

The oil containing refrigerant completing the refrigeration cycle is introduced through a suction port 35 into the cylinder head and caused to impinge on a bottom wall of the port, and the oil separated from the refrigerant is collected in an oil sump 33. The oil collected in the oil sump 33 is forcedly recovered by the gear pump 20 and moved to the oil chamber 19 during the suction stroke of the pump through a communication duct 34.

All the oil flowing into the low pressure gas passage 16 with the blow-by gas has hitherto moved through the refrigeration cycle before being separated from the gas and recovered. Thus, a considerably long time passes before the oil in the refrigerant is separated in conventional compressors. This has often caused the carryover of oil to take place.

The invention obviates all the afore-mentioned disadvantages. According to the invention, almost all the oil accompanying the blow-by gas is separated therefrom and returned to the oil chamber before being introduced into the refrigeration cycle. The efficiency of a refrigerating machine is lowered when there is a high content of oil in the refrigerant in a gaseous state. The invention minimizes the quantity of oil flowing into the refrigeration cycle. thereby increasing the efficiency of the refrigerating machine.

According to the invention. the low pressure gas passage 16 may be constructed such that. as shown in FIG. 7. the gas flows into the passage 16 at the center of the shell and flow Ieftwardly and rightwardly in two streams instead of in one direction as shown in FIG. 3. When this is the case, the oil which has not been separated from the refrigerant and introduced into the refrigeration cycle must be recovered by means other than that shown and described with reference to the aforementioned embodiment. Such means would take advantage of the fact that the low pressure gas passage 16 is bifurcated at right angles to its approach passage. No matter what means may be taken, such means should be simple in construction because the quantity of oil introduced into the refrigeration cycle is very small.

It will be appreciated that the invention offers many advantages. Since the invention is effective to separate and recover lubricant from blow-by gas, it is possible to increase the degree of efficiency with which refrigera tion is carried out. Particularly, it is possible to preclude breaking of the valves and production of abnormal noise which might otherwise occur due to overcomprcssion caused by the carry-over of oil taking place, at the time the compressor is started. It is also possible to preclude seizure and cracking of the slippers 14 and to ensure that lacking of lubricant never occurs during normal operation of the compressor. Thus seizure of the sliding parts of the inclined disc can be positively precluded.

An additional advantage offered by the invention is that the mechanical seal 23 and needle bearings 9 can be lubricated by the lubricant separated from the blowby gas according to the invention without requiring forced lubrication as shown in FIG. 3, so that the gear pump has only to lubricate the thrust bearings and the sliding parts of the inclined disc. This permits the length of the axial oil bore 22 formed in the drive shaft 8 to be reduced. Thus the bore 22 has only to extend from the rear end of the compressor to the front thrust bearing portion only as shown in the figure, making the compressor more economical than conventional compressors.

It is to be understood that the invention is not limited to the precise form of means for separating lubricant from blow-by gas described as an embodiment. and that any other means may be used instead. For example, rotary blades driven by the stream of blow-by gas may be utilized to attain the end. The lubricant separated from the blow-by gas may be either drawn by suction into the oil chamber or directly returned to the oil chamber.

FIG. 8 shows a modification of the embodiment in which, instead of projecting the shell downwardly to provide an oil sump in its lower portion, the segmental region 42 (See FIG. 2.) is used as an oil chamber (not shown). In this modification too, the oil separated in the oil separating chambers 29, 29' from the blow-by gas moves downwardly by its own weight to the oil chamber after moving through the oil return openings 31, 32 and lubricating the needle bearings 9 and mechanical seal 23. This modification is more advantageous from the points of view of production and mounting because the shell is wholly circular in its outer appearance.

It is to be understood that the invention can be incorporated not only in a compressor of the swash plate type as described above but also in all other types of compressors for refrigerating machines of the type wherein blow-by gas flows into the crank case.

What I claim is:

l. A compressor for a refrigerating machine in which rotary movements of a drive shaft are converted into reciprocatory movements of pistons in a crankcase and a refrigerant in a gaseous state is drawn by suction and compressed by a combination of pistons, cylinders, and inlet and discharge valve means. the compressor com prising passage means for maintaining the interior of the crank-case in communication with a low pressure gas passage on the suction side, and oil separating means disposed in said passage means for separating and recovering oil for lubricating the compressor from blow-by gas flowing from the crank-case to said low pressure gas passage including at least two balance holes, each of said balance holes having a small cross sectional area, and an oil separating chamber having a passage disposed between said balance holes, said passage being of a larger cross sectional area than each of said two balance holes.

2. A compressor according to claim 1, wherein oil separating chambers are disposed between the crank case and the low pressure gas passage, said at least two balance holes being disposed between said oil separating chambers and the crank-case substantially at an upper end of each of the oil separating chambers and communicating with said crank-case, and wherein at least two further balance holes are disposed between the oil separating chamber and the low pressure gas passage substantially at the upper end of each oil separating chamber and communicating with the low pressure gas passage.

3. A compressor according to claim 2, wherein the cross-sectional area of said two further balance holes is larger than the crosssectional area of said at least two balance holes.

4. A compressor according to claim 3, wherein said at least two balance holes and said two further balance holes are disposed substantially at the same level.

5. A compressor according to claim 3, wherein said two further balance holes are disposed at a level above said at least two balance holes.

6. A compressor according to claim 3, wherein the compressor includes cylinder heads, the low pressure passage communicates with low pressure chamber means provided in the cylinder heads, and wherein a suction port is provided in one of the cylinder heads for introducing the refrigerant into the compressor.

7. A compressor according to claim 3, wherein a suction port is provided for introducing the refrigerant into the compressor, said suction port being disposed substantially centrally of said two further balance holes.

8. A compressor of the swash plate type for a refrigerating machine in which an inclined disc secured to a drive shaft and pistons each slidably received in a cylinder are operatively connected to one another in a crank-case formed in a cylinder block so as to convert rotary movements of the drive shaft into reciprocatory movements of the pistons, inlet and discharge valve means for controlling gas pressure within the compressor, and a low pressure gas passage is formed to communicate with the cylinders at their suction side, such compressor comprising oil separating chambers interposed between said crank-case and said low pressure gas passage, a first set of balance holes formed between the oil separating Chambers and the crank-case and each disposed near the upper end of each oil separating chamber to maintain the oil separating chambers in communication with the crank-case, and a second set of balance holes formed between the oil separating chambers and the low pressure gas passage and each disposed near the upper end of each oil separating chamber to maintain the oil separating chambers in communication with the low pressure gas passage, said oil separating chambers each being formed with an oil return opening formed at a lower portion thereof.

9. A compressor of the swash plate type for a refrigerating machine according to claim 8 in which said oil separating chambers provided between the crank-case and the low pressure gas passage are closed and equal to each other in dimensions. said balance holes of the first set maintaining an upper portion of the crank-case in communication with upper portions of the oil separating chambers are small in size, and said balance holes of the second set formed between the oil separating chambers and the low pressure gas passage to maintain them in communication with one another are disposed in upper portions of the oil separating chambers in positions in which they are spaced apart from the balance holes of the first set so as to greatly vary the directions of streams of the refrigerant in a gaseous state 10. A compressor of the swash plate type according to claim 9 further comprising a cylinder block consisting of a pair of cylinder block elements of equal size and shape and built in a shell, a pair of cylinder heads each cylinder head being secured to one of opposite ends of said cylinder block through a valve plate interposed therebctween, and low pressure chambers each defined in one of said cylinder heads, said low pressure gas passage being formed in a portion of said cylinder block in which no cylinders are formed and which is formed with a recess so that the low pressure gas passage is defined by a bottom and walls of the recess and the shell, said oil separating chambers being formed in spaces formed below opposite end portions of said recess with partitions being disposed between said spaces and said low pressure gas passage so that each said oil separating chamber is defined between one of the valve plates and one outer wall of the crank-case, said valve plates each being formed therein with a suction port so as to maintain each of said low pressure chambers in communication with the low pressure gas passage, and said second set of balance holes being disposed in close proximity to said valve plates.

11. A compressor of the swash plate type for a refrigerating machine according to claim 8 wherein one of said oil return openings formed in the lower portions of said oil separating chambers is maintained in communication with a needle bearing portion arranged between the drive shaft and the cylinder block and the other oil return opening is maintained in communication with a mechanical seal portion arranged near one end of the drive shaft. 

1. A compressor for a refrigerating machine in which rotary movements of a drive shaft are converted into reciprocatory movements of pistons in a crankcase and a refrigerant in a gaseous state is drawn by suction and compressed by a combination of pistons, cylinders, and inlet and discharge valve means, the compressor comprising passage means for maintaining the interior of the crank-case in communication with a low pressure gas passage on the suction side, and oil separating means disposed in said passage means for separating and recovering oil for lubricating the compressor from blow-by gas flowing from the crank-case to said low pressure gas passage including at least two balance holes, each of said balance holes having a small cross sectional area, and an oil separating chamber having a passage disposed between said balance holes, said passage being of a larger cross sectional area than each of said two balance holes.
 2. A compressor according to claim 1, wherein oil separating chambers are disposed between the crank-case and the low pressure gas passage, said at least two balance holes being disposed between said oil separating chambers and the crank-case substantially at an upper end of each of the oil separating chambers and communicating with said crank-case, and wherein at least two further balance holes are disposed between the oil separating chamber and the low pressure gas passage substantially at the upper end of each oil separating chamber and communicating with the low pressure gas passage.
 3. A compressor according to claim 2, wherein the cross-sectional area of said two further balance holes is larger than the cross-sectional area of said at least two balance holes.
 4. A compressor according to claim 3, wherein said at least two balance holes and said two further balance holes are disposed substantially at the same level.
 5. A compressor according to claim 3, wherein said two further balance holes are disposed at a level above said at least two balance holes.
 6. A compressor according to claim 3, wherein the compressor includes cylinder heads, the low pressure passage communicates with low pressure chamber means provided in the cylinder heads, and wherein a suction port is provided in one of the cylinder heads for introducing the refrigerant into the compressor.
 7. A compressor according to claim 3, wherein a suction port is provided for introducing the refrigerant into the compressor, said suction port being disposed substantially centrally of said two further balance holes.
 8. A compressor of the swash plate type for a refrigerating machine in which an inclined disc secured to a drive shaft and pistons each slidably received in a cylinder are operatively connected to one another in a crank-case formed in a cylinder block so as to convert rotary movements of the drive shaft into reciprocatory movements of the pistons, inlet and discharge valve means for controlling gas pressure within the compressor, and a low pressure gas passage is formed to communicate with the cylinders at their suction side, such compressor comprising oil separating chambers interposed between said crank-case and said low pressure gas passage, a first set of balance holes formed between the oil separating chambers and the crank-case and each disposed near the upper end of each oil separating chamber to maintain the oil separating chambers in communication with the crank-case, and a second set of balance holes formed between the oil separating chambers and the low pressure gas passage and each disposed near the upper end of each oil separating chamber to maintain the oil separating chambers in communication with the low pressure gas passage, said oil separating chambers each being formed with an oil return opening formed at a lower portion thereof.
 9. A compressor of the swash plate type for a refrigerating machine according to claim 8 in which said oil separating chambers provided between the crank-case and the low pressure gas passage are closed and equal to each other in dimensions, said balance holes of the first set maintaining an upper portion of the crank-case in communication with upper portions of the oil separating chambers are small in size, and said balance holes of the second set formed between the oil separating chambers and the low pressure gas passage to maintain them in communication with one another are disposed in upper portions of the oil separating chambers in positions in which they are spaced apart from the balance holes of the first set so as to greatly vary the directions of streams of the refrigerant in a gaseous state.
 10. A compressor of the swash plate type according to claim 9 further comprising a cylinder block consisting of a pair of cylinder block elements of equal size and shape and built in a shell, a pair of cylinder heads each cylinder head being secured to one of opposite ends of said cylinder block through a valve plate interposed therebetween, and low pressure chambers each defined in one of said cylinder heads, said low pressure gas passage being formed in a portion of said cylinder block in which no cylinders are formed and which is formed with a recess so that the low pressure gas passage is defined by a bottom and walls of the recess and the shell, said oil separating chambers being formed in spaces formed below opposite end portions of said recess with partitions being disposed between said spaces and said low pressure gas passage so that each said oil separating chamber is defined between one of the valve plates and one outer wall of the crank-case, said valve plates each being formed therein with a suction port so as to maintain each of said low pressure chambers in communication with the low pressure gas passage, and said second set of balance holes being disposed in close proximity to said valve plates.
 11. A compressor of the swash plate type for a refrigerating machine according to claim 8 wherein one of said oil return openings formed in the lower portions of said oil separating chambers is maintained in communication with a needle bearing portion arranged between the drive shaft and the cylinder block and the other oil return opening is maintained in communication with a mechanical seal portion arranged near one end of the drive shaft. 